The use of spread spectrum communications techniques to improve the reliability and security of communications is well known and is becoming increasingly common. Spread spectrum communications transmits data utilizing a spectrum bandwidth that is much greater than the bandwidth of the data to be transmitted. This provides for a more reliable communication in the presence of high narrowband noise, spectral distortion and pulse noise, in addition to other advantages. Spread spectrum communication systems typically utilize correlation techniques to identify an incoming received signal.
Spread spectrum communications systems were used for use in military environments to overcome high-energy narrowband enemy jamming. In commercial or home environments, they may be used to achieve reliable communication on noise media such as the AC power line. In particular, certain home electrical appliances and devices can potentially be very disruptive of communications signals placed onto the power line. For example, electronic dimming devices can place large amounts of noise onto the power line since these devices typically employ triacs or silicon controlled rectifiers (SCRs) to control the AC waveform in implementing the dimming function.
A communication medium such as the AC power line may be corrupted by fast fading, unpredictable amplitude and phase distortion and additive noise. In addition, communication channels may be subjected to unpredictable time varying jamming and narrowband interference. In order to transmit digital data over such channels it is preferable to use as wide a bandwidth as possible for transmission of the data. This can be achieved using spread spectrum techniques.
The spread spectrum receiver is required to perform synchronization that is commonly achieved using some form of acquisition method optionally in combination with a tracking loop or other tracking mechanism. In a noisy unpredictable environment such as the AC power line, the tracking loop typically fails frequently causing loss of information. Communication systems to overcome these problems are large, complex and expensive.
Synchronization of signals between the transmitter and the receiver that are communicating with each other in a spread spectrum communication system is an important aspect of the process of transmitting signals between them. Synchronization between transmitter and receiver is necessary to allow the despreading of the received signals using a spreading code that is synchronized between them so that the originally transmitted signal can be recovered from the received signal. Synchronization is achieved when the received signal is accurately timed in both its spreading code pattern position and its rate of chip generation with respect to the receiver's spreading code.
One of the problems associated with synchronization is that the techniques used to synchronize two signals are relatively expensive to implement. In communication systems having sophisticated and relatively expensive central communication sites which serve a plurality of relatively inexpensive remote communication sites, it is desirable to reduce the cost of synchronization systems in the remote communication sites while not increasing the cost of the central communication sites.
In a communications transceiver, it is desirable that the acquisition mechanism be more reliable that any error correction code used for the data portion of the packet. In other words, it is preferable to declare synchronization correctly and not be able to correctly decode the packet data than to miss the entire packet altogether because of a weak acquisition algorithm.
Further, it is desirable that the acquisition algorithm has as low a probability as possible of false synchronization from noise, e.g., less than once in 5 seconds. The acquisition mechanism should be capable of utilizing more than one synchronization sequence whereby the probability of synchronization from another sequence is minimized.